1. Field of the Invention
The present invention relates to a longitudinal magnetic recording medium with noise suppressed and stability improved, and more particularly to a magnetic recording apparatus having a high recording density which is arranged to use the longitudinal magnetic recording medium.
2. Description of the Related Art
In recent days, a request has been increasingly elevated for enlarging a volume of a magnetic disk drive. Accordingly, the magnetic head has been requested to have a far higher efficiency and the recording medium has been requested to have a far higher coercivity and lower noise.
The magnetic head is used of a composite head that includes both an inductive head for recording data and a spin-valve type head for reading back data. The spin-valve type head is a read-back head that is composed of a magnetoresistive sensor having a plurality of conductive magnetic layers whose directions of magnetization are relatively changed by the outside magnetic field so that a large resistance change may be brought about and conductive non-magnetic layers located between the adjacent conductive magnetic layers.
The magnetic recording medium is composed of a first underlayer called a seed layer formed on a substrate, a second underlayer composed of a Cr alloy having a body-centered cubic structure (bcc structure), a magnetic layer composed of a Co alloy having a hexagonal closed packed structure, and a carbon protective layer. In order to obtain a strong in-plane magnetic anisotropy (high in-plane coercivity), it is preferable that the longitudinal magnetic recording medium has a c-axis, that is, an axis of easy magnetization of the magnetic layer is oriented into the in-plane direction. Hence, the Co alloy of the magnetic layer has an orientation in which the (11.0) plane is positioned in parallel to the substrate plane (called the (11.0) orientation) or another orientation in which the (10.0) plane is positioned in parallel to the substrate plane (called the (10.0) orientation). It is known that the crystal lattice of the magnetic layer may be controlled by the seed layer. Further, it has been reported that the former orientation can be obtained by using Ta (see JP-A-4-188427) or MgO (see Appl. Phys, Lett., vol. 67, pp. 3638-3640, December (1993)) for the seed layer and the latter orientation can be obtained by using an NiAl alloy having a B2 crystal structure (see IEEE Trans. Magns., vol 30, pp. 3951 to 3953 (1994)) for the seed layer.
In order to further enhance the orientation of the magnetic layer, it has been studied that a non-magnetic Co alloy having a hcp structure is formed as a third underlayer between the second underlayer composed of a Cr alloy and the magnetic layer composed of a Co alloy. This study is tried as remarking the fact that the crystal of the magnetic layer is grown on the Co alloy underlayer having the same hcp structure as that of the magnetic layer more microfine than on the Cr alloy underlayer having a bcc structure. As this type of example, the CoCr alloy (see JP-A-10-79113 or JP-A-10-233014) or the CoCrRu alloy (see JP-A-2000-113445) has been reported.
It is a first object of the present invention to provide a magnetic recording medium having a longitudinal recording density of 30 megabits or more per one square millimeter, which has a low noise and high coercivity, and is sufficiently stable for thermal fluctuation.
The inventors tried the following experiment. A non-magnetic alloy layer composed of a Co-40 at. % Ru alloy having a hexagonal closed packed structure (hcp structure) was laid between the magnetic layer and the Cr alloy underlayer. Then, the change of the characteristic was studied. As a result, when the Cr alloy underlayer has (100) orientation and the average grain size of the underlayer is as small as 20 to 25 nm or less, it was found that the characteristics such as the coercivity and reduction of noise are remarkably improved.
The magnetic recording medium of the first object has the following structure. That is, an amorphous first underlayer, a second underlayer of a body-centered cubic structure having Cr as a main component, a third underlayer of a hexagonal closed packed structure having Co as a main component, a magnetic layer of a hexagonal closed pack structure, and a carbon protective film, all of which are formed on the non-magnetic substrate vertically in this describing sequence. Then, a lubricant agent is coated on the carbon protective film. Herein, the term xe2x80x9camorphousxe2x80x9d means that no obvious diffraction peak indicates that except a hollow pattern appears in the X-ray diffraction spectrum or the average grain size obtained from a lattice image imaged by a high resolution electronic microscope is 5 nm or less.
In order to keep the second underlayer of the body-centered cubic structure having Cr as a main component in the (100) orientation and make the average grain size smaller, it is preferable to form the first underlayer of the following amorphous alloy, in which alloy Cr is used as a main component and at least one element selected from the first element group consisting of Cr, V and Mn constitutes 30 at. % to 60 at. % and at least one element selected from the second element group consisting of Zr, Hf, Ta, Nb, Ti, W, Mo, B and Si constitutes 3 at. % to 30 at. %. If the total sum of the addition of the first element group is 30 at. % or less, the magnetization is not sufficiently cancelled, while if the total sum of the addition of the second element group is 60 at. % or more, undesirably, it is difficult to implement the amorphousness. Further, if the total sum of the addition of the second element group ranges from 3 at. % to 30 at. %, undesirably, the amorphousness cannot be implemented. The use of the amorphous Co alloy on the first underlayer makes it possible to become the grain size of the magnetic layer smaller, which is preferable to obtaining the medium with reduced noise.
As another method, it is possible to use, as the first underlayer, the following amorphous alloy in which Ni is used as a main component, at least one element selected from the foregoing first element group constitutes 50 at. % or less, and at least one element selected from the third element group consisting of Zr, Ta, Ti, W, Mo, B and Si constitutes 3 at. % to 60 at. %. In this case, since the magnetic layer has an especially strong (11.0) orientation, this amorphous alloy is preferable to obtaining a medium with high coercivity. If the addition of the element selected from the third element group causes the underlayer of the Ni alloy to be sufficiently non-magnetized, no addition of the first element group is required. If non-magnetization is not sufficient, it is necessary to add at least one element of the first element group. In order to prevent the underlayer of the Ni alloy from being crystallized, it is preferable to suppress the total sum of the addition to 50 at. % or less. Further, it is preferable to suppress the total sum of the addition of the third element group to 3 at. % to 60 at. % for the purpose of preventing the underlayer from being crystallized.
The alloy used for making the first underlayer is not limited only if it is amorphous and has a microfine crystal structure having an average crystal grain size of 5 nm or less. It was assured that the same effect in improving the characteristics as the above can be obtained by using amorphous a Cr-15 at. % Ti or Nb-15 at. % Si alloy for the first underlayer. Though it is preferable that the first underlayer is non-magnetic, if Br1xc2x7t1 (a product of residual magnetic flux density Br1 and a film thickness t1 of the first underlayer) is 20% or less of Brxc2x7tmag (a product of residual magnetic flux density Br and a film thickness tmag of the magnetic layer), no substantial problem takes place even if some magnetization is left.
Further, after forming the first underlayer, by exposing the first underlayer in a mixed gas atmosphere having argon as its main component where oxygen constitutes 1 to 10% for several seconds for the purpose of artificially oxidizing the surface of the first underlayer, it is possible to make the grain size of the second underlayer smaller. In this case, the grain size of the magnetic layer is also made smaller. Hence, the resulting medium has a far lower noise characteristic. This process is especially effective in the case of forming a film with a leaf type sputtering device having a high throughput in the condition that the vacuum degree of the base is roughly 7xc3x9710xe2x88x925 Pa or less, or the time taken between the formation of the first underlayer and that of the second underlayer is roughly 20 seconds or less. As another method, in a case that the first underlayer is formed in a mixed gas atmosphere having argon as its new component and an 1 to 10% oxygen, the same effect as the case of introducing the foregoing process of oxidizing the surface can be obtained. Though it is preferable to keep the substrate temperature at the time of forming the first underlayer to a room temperature, since the foregoing alloy material is made amorphous in the temperature of 100 to 200xc2x0 C. or less, it is possible to heat the substrate for the purpose of degassing the substrate. Though the thickness of the first underlayer is not limited so much, it is preferable to keep it in the range of 20 nm to 100 nm in consideration of unique heating on the substrate and the crystallization caused by increasing the film thickness.
It is possible to use the second underlayer composed of a Cr alloy having a bcc structure in which Cr is used as a main component and Mo, W, V, Nb and Ta are included. In particular, preferably in the case of using a CrTi alloy in which Ti constitutes 3 at. % to 35 at. %, the resulting medium has a high coercivity and a low noise characteristic.
It is possible to use as the third underlayer an alloy material having Co as a main component and a 35 at. % to 60 at. % Ru. Since the Ru is larger in an atomic size than Co, the Coxe2x80x94Ru underlayer is especially suitable to the magnetic layer having a massive amount of Pt, which is as large as 12 to 14 at. %. If the content of Ru is less than 35 at. %, the magnetization is not sufficiently reduced, while if it is more than 65 at. %, the (100) orientation of the underlayer is broken. Hence, neither of the cases are preferable. Though it is preferable to make the third underlayer non-magnetic, if Br3xc2x7t3 (a product of residual magnetic flux density Br3 and a film thickness t3 of the third underlayer) is 20% or less of Brxc2x7tmag (a product of residual magnetic flux density Br and a film thickness tmag of the magnetic layer), no practical problem takes place even if some magnetization is left.
By adding an 1 at. % to 12 at. % B to the third underlayer having Co as its main component, the grain size of the underlayer is made uniform. This makes it possible to suppress the occurrence of so microfine a gain as 2 nm to 3 nm in the magnetic layer. Though the micro magnetic crystal grain is strongly suffered from the adverse effect of the thermal fluctuation, the thermal stability is improved by excluding these micro crystal grains. If the total sum of the addition of the foregoing elements is less than 1 at. %, the effect of making the grain size uniform is not sufficient, while if it is more than 12 at. %, the hcp structure of the third underlayer is broken. Hence, neither of the cases are preferable.
The magnetic layer may be composed of an alloy having Co as its main component such as a CoCrPtB alloy or a CoCrPtTaB alloy, that is, an alloy described in Co100-a-b-c-dCraPtbBcTad (16 at. %xe2x89xa6axe2x89xa622 at. %, 12 at. %xe2x89xa6bxe2x89xa618 at. %, 4 at. %xe2x89xa6cxe2x89xa612 at. %, 0 at. %xe2x89xa6d xe2x89xa63 at. %). In order to reduce the exchange interaction between the magnetic grains and obtain a high read output, it is preferable to limit the content of Cr in the range of 16 at. % to 22 at. %. Further, in order to obtain high crystal magnetic anisotropy and an excellent overwrite characteristic, it is preferable to limit the content of Pt in the range of 12 at. % to 18 at. %. Further, in order to make the magnetic grain size smaller and thereby reduce the medium noise, it is preferable to include a 4 at. % to 12 at. % B. If B is more than 12 at. %, the hcp structure of the magnetic layer is broken. Further, the effect caused by making the grain size smaller is not sufficient if it is less than 4 at. %. Hence, neither of the cases are preferable. If Ta constitutes 3 at. % or less, the medium noise can be reduced without breaking the hcp structure of the magnetic layer. Hence, it is preferable to obtain the medium with a low noise characteristic.
As to the magnetic characteristic of the magnetic layer, it is preferable to keep the coercivity in the range of 270 kA/m (3402 oersted) to 360 kA/m (4536 oersted) and Brxc2x7tmag (a product of residual magnetic flux density Br and a film thickness tmag of the magnetic layer) in the range of 3.0 Txc2x7nm (30 Gxcexcm) to 7.0 Txc2x7nm (70 Gxcexcm). If the coercivity does not reach 270 kA/m, the recording resolution is made lower, while if it exceeds 360 kA/m, the overwrite characteristic is made degraded. Hence, neither of them are preferable. Further, if Brxc2x7tmag is less than 3.0 Txc2x7nm, the read output is made lower, while if it exceeds 7.0 Txc2x7nm, the resolution is made lower. Hence, neither of the cases are preferable. Further, in order to keep sufficient stability to thermal fluctuation, it is preferable to make a thermal stability factor (Kuxc2x7v/kT) 90 or higher (Ku: crystal magnetic anisotropy constant, v: magnetic grain volume, k: Boltzmann constant, T: absolute temperature). The thermal stability factor can be measured by fitting the time dependency of the residual coercivity to the Sharrock expression. According to the inventors"" study, if the Kuxc2x7v/kT in the room temperature derived by this method is 90 or higher, the decay of the read output after five years is estimated as 10% or less. Hence, no problem on the reliability takes place. Further, though it is preferable to keep the magnetic layer in the (11.0) orientation, if the crystal grains in another orientation exist, no special problem takes place even if the diffraction peak strength from the crystal orientation plane in the X-ray diffraction spectrum is lower than the (11.0) diffraction peak strength.
Further, by forming the carbon with an addition of nitrogen as a protective film in the thickness of 3 nm to 7 nm and coating a lubricant layer composed of absorptive perfluoroalkyl-polyether in the thickness of 1 nm to 4 nm, it is possible to form the magnetic recording medium that is highly reliable and has a high recording density. The use of a carbon film with an addition of hydrogen as a protective layer, a film composed of a compound such as silicon carbide, tungsten carbide, (Wxe2x80x94Mo)xe2x80x94C or (Zrxe2x80x94Nb)xe2x80x94N, or a mixed film of these compounds and carbon results in improving durability and corrosion resistance.
It is a second object of the present invention to provide a magnetic recording apparatus using the magnetic recording medium described above.
The highly reliable magnetic recording apparatus of the second object has a magnetic recording medium described in the first object, a driving unit for driving the medium in the recording direction, a magnetic head composed of a recording unit and a read-back unit, a unit for moving the magnetic head relatively against the magnetic recording medium, and a read/write signal processing unit for inputting a signal into the magnetic head and reading back an output signal from the magnetic head, characterized by using any one of the foregoing media as the magnetic recording medium so that the in-plane recording density reaches 30 megabits or more per one square millimeter. The read-back section of the magnetic head is composed of a spin-valve sensor having a plurality of conductive magnetic layers whose magnetizing directions are relatively changed with respect to one another so that a large resistance change may take place and conductive non-magnetic layers positioned between the adjacent conductive magnetic layers. It is preferable that the sensor unit is formed between two shield layers composed of a soft magnetic material, spaced from each other by 0.15 xcexcm or less. This is because if the interval between the shields is 0.15 xcexcm or more, the resolution is made lower and the phase jitter of the signal is made larger. By arranging the storage device as described above, it is possible to implement a highly reliable magnetic recording apparatus having a far higher signal strength and a recording density of 30 megabits or more per one square millimeter.
It is a third object of the present invention to provide a magnetic recording medium having a longitudinal recording density of 35 megabits or more per one square millimeter.
The magnetic recording medium of the third object has a high S/N ratio and coercivity and being sufficiently stable to thermal fluctuation. A second underlayer of the recording medium uses an alloy containing Cr as a main component and Ti, and Mo or W, thereby realizing a high reliable magnetic recording medium of having 35 megabits or more per one square millimeter as a longitudinal recording density.
The magnetic recording medium of the third object has the following structure. That is, an amorphous first underlayer is formed on the non-magnetic substrate, a second underlayer of the b.c.c. structure having Cr as its main component is formed on the first underlayer, a third underlayer of the h.c.p structure having Co as its main component is then formed on the second underlayer and formed a magnetic layer of a the h.c.p structure thereon, and a protective layer having C as its main component is finally formed thereon. Then, a lubricant agent is coated on the protective layer. The sectional structure is epitaxially grown by performing the lattice matching of the (200) plane of the b.c.c structure of the second underlayer, the (11.0) plane of the h.c.p structure of the third underlayer, and the (11.0) plane of the h.c.p structure of the magnetic layer on their layer interfaces.
In the case of using a Coxe2x80x94Crxe2x80x94Ptxe2x80x94B alloy for the magnetic layer, a Coxe2x80x94Ru alloy for the third underlayer or the underlayer composed of a Crxe2x80x94Ti alloy having no Mo or W, the lattice constant of the second underlayer is smaller than that of the third underlayer. Hence, the epitaxial growth of the third underlayer on the second underlayer is made difficult, so that the sufficient (11.0) orientation of the magnetic layer cannot be obtained. Further, since the Crxe2x80x94Ti alloy film is made smaller in crystal grains by restricting the content of Ti to about 20 at. %, the second underlayer is an essential element for obtaining the first and second objects of the invention (see J. Appl. Phys. 79, pp. 5351 to 5353 (1996)). However, with increase of the content of Ti, the lattice constant is made larger, while if the content of Ti exceeds 20% to 25%, the increase of the crystal grain size causes the increase of the noise on the medium. Hence, using the underlayer composed of the Crxe2x80x94Ti alloy does not realize a magnetic recording medium having 35 megabits or more per one square millimeter.
Hence, the inventors have found the method of increasing the lattice constant by adding Mo or W to the Crxe2x80x94Ti alloy and thereby enhancing the lattice matching with the third underlayer (Coxe2x80x94Ru matching layer). As a result, in actual, a lamination of the second underlayer (composed of the Crxe2x80x94Tixe2x80x94Mo alloy) and the third underlayer (a matching layer of the Coxe2x80x94Ru alloy) or a lamination of the second underlayer (composed of the Crxe2x80x94Tixe2x80x94W alloy) and the third underlayer (a matching layer of the Coxe2x80x94Ru alloy) make it possible to improve the (11.0) orientation of the magnetic layer, in particular, remarkably enhance the coercivity and the S/N ratio, enhancing a longitudinal recording density moreover.
As to the non-magnetic substrate 10 may be used alumino silicate, ceramics compose of soda lime glass, silicon, borosilicate glass or the like, an Alxe2x80x94Mg alloy substrate on which Nixe2x80x94P is electroless-plated, a rigid substrate composed of glass on which Nixe2x80x94P is electroless-plated, or the like.
In order to keep the second underlayer of the b.c.c. structure in the (200) orientation and make the average crystal grain size smaller, it is possible to use an amorphous alloy having Co as its main component and containing elements of Cr and Zr for the first underlayer. At this time, if the addition of Cr is less than 30 at. %, the magnetization cannot be sufficiently cancelled, while if it is more than 60 at. %, the amorphousness becomes difficult. Neither of the cases are preferable. It was assured that if the addition of Zr is 5 at. % to 15 at. %, the amorphousness is realized, the (200) orientation of the second underlayer is also realized, and the average crystal grain size is made smaller. As to the amorphous Co alloy used for the first underlayer, the Coxe2x80x94Crxe2x80x94Ta alloy or the Coxe2x80x94Crxe2x80x94W alloy having the same composition ratio is effective. Further, the first underlayer may use an amorphous alloy having Ni as its main component and containing the elements of Cr and Zr. In this case, if the addition of Cr is less than 20 at. %, the magnetization cannot be completely cancelled, while if it is more than 60 at. %, the amorphousness is made difficult. Hence, neither of the cases are preferable. When the addition of Zr is 5 at. % to 15 at. %, the amorphousness is achieved. It was assured that the (200) orientation of the second underlayer is realized and the average crystal grain size is made smaller. As the amorphous Ni alloy is used an Nixe2x80x94Ta alloy (the content of Ta is 35 to 40 at. %), an Nixe2x80x94Taxe2x80x94Zr alloy (the content of Ta is 35 to 40 at. % and the content of Zr is 5 to 15 at. %), and an Nixe2x80x94Nbxe2x80x94Zr alloy (the content of Nb is 20 to 40 at. % and the content of Zr is 5 to 15 at. %). Further, the stiffness or Young""s modulus may be adjusted by laminating the amorphous Co alloy film on the amorphous Ni alloy film for forming the first substrate. In this case, no substantial problem takes place in the foregoing effects.
Herein, it is preferable that the first underlayer is non-magnetic. However, if Br1xc2x7t1 (a product of residual magnetic flux density Br1 and a film thickness t1 of the first underlayer) is less than Brxc2x7tmag (a product of residual magnetic flux density Br and a film thickness tmag of the magnetic layer) by 20% or less, no substantial problem takes place even if some magnetization is left.
Further, by forming the first underlayer and exposing the first underlayer in a mixed gas atmosphere having argon as its main component and 1 to 10% oxygen added, argon for several seconds, for the purpose of artificially oxidizing the surface of the underlayer, as heating the underlayer up to 200 to 300xc2x0 C. or after heated, the grain size of the second underlayer can be made smaller. In this case, the grain size of the magnetic layer is made more microfine, which leads to suppressing the noise of the medium. The introduction of the oxidizing process is quite effective in the case of using the sputtering device having its basic vacuum of about 7xc3x9710xe2x88x925 Pa or less for forming a film. Further, preferably, the substrate temperature required in forming the first underlayer should be a room temperature. However, if the substrate temperature ranges from 100 to 200xc2x0 C., the alloy material is made amorphous. Hence, the substrate may be heated for the purpose of degassing the substrate. The thickness of the first underlayer is not limited especially. In actual, the thickness thereof should be 10 nm to 50 nm in consideration of the uniform heating of the substrate and the crystallization caused by the increase of the thickness.
The second underlayer may be used of an alloy having Cr as its main component and containing a 10 to 25 at. % Ti or a 2 to 20 at. % Mo or W. If the content of Ti is less than 10 at. %, the crystal matching to the third underlayer composed of a Coxe2x80x94Ru alloy formed thereon is made lower and the orientation of the magnetic layer is made degraded accordingly. Hence, the case is not preferable. On the other hand, if the content of Ti exceeds 25 at. %, the crystal grain size of the second underlayer is increased and thereby the medium noise is increased. Further, the content of Mo or W to be added to the second underlayer should be 2 to 20 at. %, because the content makes the crystal matching excellent. By adding an 1 to 10 at. % B to the second underlayer, preferably, the grain size of the second underlayer may be made uniform.
The third underlayer may be used a Co alloy material containing a 35 to 60 at. % Ru. If the content of Ru is less than 35 at. %, the magnetization can be completely reduced, while if it is more than 60 at. %, the crystal matching to the magnetic layer may be made lower. Hence, neither of the cases are preferable. Though it is preferable to make the third underlayer non-magnetic, if Br3xc2x7t3 (a product of residual magnetic flux density Br3 and a film thickness t3 of the third underlayer) is equal to or less than 20% of Brxc2x7tmag (a product of residual magnetic flux density Br and a film thickness tmag of the magnetic layer), no practical problem takes place if some magnetization is left.
The magnetic layer may be composed of a Coxe2x80x94Crxe2x80x94Prxe2x80x94B alloy having Co as its main component in which a Cr concentration is 16 to 22 at. %, a Pt concentration is 12 to 18 at. %, a B concentration is 4 to 12 at. %, and the remaining portion is Co. In order to reduce the exchange interaction among the magnetic grains and obtain a high read output, it is preferable to make the content of Cr range from 16 to 22 at. %. In order to obtain high crystal magnetic anisotropy and an excellent overwrite characteristic, it is preferable to make the content of Pt the range from 12 to 18 at. %. Further, by making the magnetic grain size small and thereby reducing the medium noise, it is preferable to make the content of B range from 4 to 12 at. %. If the content of B exceeds 12 at. %, undesirably, the h.c.p structure of the magnetic layer is broken.
As to the magnetic characteristic of the magnetic layer, the coercivity should be 300 kA/m (3770 oersted) to 370 kA/m (4650 oersted) and the Brxc2x7tmag should be 3.0 Txc2x7nm (30 G xcexcm) to 7.0 Txc2x7nm (70 xcexcm). If the coercivity does not reach 300 kA/m, the recording resolution is made lower, while if it exceeds 370 kA/m, the overwrite characteristic is made degraded. Hence, neither of the cases are preferable. Further, if Brxc2x7tmag is less than 3.0 Txc2x7nm, the read output is made lower, while if it exceeds 7.0 Txc2x7nm, the resolution is made lower. Hence, neither of the cases are preferable. Moreover, in order to keep the sufficient stability to the thermal fluctuation, it is preferable to enlarge a thermal stability factor kuV/KT (Ku: crystal magnetic anisotropy constant, V: magnetic grain volume, k: Boltzmann constant, T: absolute temperature). Though the thermal stability factor is variable according to the various measuring methods, as disclosed in J Magn. Magn. Mater. 127, pp. 233 (1993), the time dependency on the residual coercivity may be measured by fitting it to the Sharrock expression. According to the inventors"" study, if KuV/kT in the room temperature derived by this method is 90 or more, the decay of the read output after five years may be estimated to be 10% or less and thus no problem on the reliability was found. Further, though the magnetic layer should have the (11.0) orientation, even if the other crystal grains have another crystal orientation, no special problem was found only if the diffraction peak intensity from the crystal orientation plane in the X-ray diffraction spectrum is lower than the (11.0) diffraction peak intensity.
By forming as a protective layer carbon having an addition of nitrogen in the thickness of 3 to 6 nm and coating a lubricant layer of absorptive perfluoroalkyl polyether in the thickness of 1 to 2 nm, the resulting magnetic recording medium is made to be highly reliable and to have a high recording density. Further, the use of a carbon film with an addition of hydrogen, a film composed of a compound such as silicon carbide, or a mixed film of the compound and the carbon, makes it possible to disadvantageously improve the durability and corrosion resistance.
By mounting those media described above to the magnetic recording apparatus described with respect to the second object of the invention, it is possible to implement a highly reliable magnetic recording apparatus having a recording density of 35 megabits or more per one square millimeter.